Method of, and system for, reserving timeslots in a tdma system

ABSTRACT

A method of, and system for, reserving timeslots in a TDMA system of a type in which a plurality of subscriber units (CPE) use the same radio channel for transmission during different timeslots to a base station and in which the base station administers the radio channel centrally enable subscriber units to transfer, if appropriate, a transmission request to the base station in the form of a PN (Pseudo-random noise) sequence (e.g., M-sequence, preferred gold, Katsami or orthogonal gold sequence). Transmission requests from different subscriber units can be identified at the base station using the PN sequence, reception time of the PN sequence and/or phase of the PN sequence.

[0001] The present invention relates to a method of, and system for,reserving timeslots in a TDMA (time-division multiple access) system, inwhich a plurality of subscriber units use the same radio channel fortransmission during different timeslots, and a base station administersthe radio channel centrally.

[0002] Systems of the generic type and methods of the generic type areused in TDMA systems, in particular TDMA point-to-multipoint radiosystems (PMP), to assign a subscriber unit ((CPE) customer premisesequipment) a chronologically limited range of a radio channel fortransmission. In TDMA multipoint systems, a plurality of subscribers usethe same radio channel for transmission in chronological succession inthe uplink, that is to say when transmitting from the subscriber unit tothe base station. Since data is transmitted in a burst-like fashion, itis particularly inefficient to transmit small data quantities, forexample a reservation request alone, as a preamble or a midamble isalmost always necessary for synchronization in the receiver when thedata is transmitted. The actual transmitted information, that is to sayfor example the reservation request, then takes up only a small portionof the total amount of data transmitted so that when a small number ofbits is transmitted in this way the spectral efficiency is particularlylow.

[0003] The base station which administers the radio channel centrallyassigns a subscriber a timeslot for transmission in the uplink, via acollision-free downlink when necessary.

[0004] As the time of a new request for a transmission timeslot cannotgenerally be predicted, it has already been proposed to perform a fixedassignment of timeslots so that each subscriber unit has thepossibility, at specific time intervals, of transmitting a reservationrequest to the base station in the uplink. However, a fixed assignmentof timeslots utilizes the transmission channel inefficiently as therewill be a large number of permanently allocated timeslots in which thesubscriber units do not output a reservation request.

[0005] In addition to the fixed assignment of timeslots, it has alsobeen proposed to interrogate the subscriber units at regular intervals.This method is termed polling. If a subscriber unit is polled, atimeslot is allocated to it in which it can transmit data, that is tosay both reservation requests and subscriber data. However, in order toachieve a short transmission delay, the subscriber units must beinterrogated very frequently so that polling is also wasteful oftransmission capacity.

[0006] It is also known to assign timeslots on the basis of what arereferred to as random access timeslots. These are timeslots which arenot assigned permanently to a subscriber unit. Instead, a plurality ofsubscribers can dispatch a reservation request in the timeslot whennecessary. However, it is disadvantageous that data is lost when thereis multiple access to the random access timeslot, that is to say whenthere is a collision. In the case of a collision, the subscriber unitsmust transmit again later. However, because the possibility ofcollisions repeatedly occurring is not excluded, long transmissiondelays are possible in a system using random access timeslots. In orderto limit these transmission delays it has been proposed, after acollision occurs, to increase the number of random access timeslotsand/or restrict the number of transmission-authorized subscriber units.However, this has the disadvantage that when there are smalltransmission delays such a method for resolving collisions leads towastage of the transmission bandwidth.

[0007] According to the present invention there is provided a method ofreserving timeslots in a TDMA system comprising a plurality ofsubscriber units (CPE) that use the same radio channel for transmissionduring different timeslots to a base station, and in which the basestation administers the radio channel centrally, which is characterizedby: if appropriate, the subscriber units transferring a transmissionrequest to the base station in the form of a PN (pseudo- random noise)sequence.

[0008] Here, it is, for example, possible to carry out ON/OFFsignalling. Detection of the PN sequence of a subscriber unit by thebase stations indicates that data is present for transmission in thesubscriber unit. If no sequence is detected, this is taken as indicatingthat the subscriber unit does not request an uplink connection. Sincethe base station is capable of identifying subscriber units from the PNsequence, this prevents collisions occurring. If a PN sequence isdetected, it is possible, for example in the next MAC (medium accesscontrol) frame, to allocate a timeslot for the transmission of data.More precise reservation data can then be transmitted with this using a“piggyback” method. It is also possible for a dedicated control timeslotto be allocated in the form of a short burst via which the onlyinformation transmitted then is a detailed reservation request. In allcases, it is possible using the method of the invention to keep the timebetween the subscriber unit dispatching the reservation request in theform of a PN sequence up to the availability of, for example, detailedfilling level information in a buffer of a subscriber unit, virtuallyconstant, for example of a length of two time periods of an MAC frame.

[0009] The use of PN sequences in data communication has already beensuccessfully implemented in CDMA systems. In CDMA systems, the actualtransmission data can be encoded on a subscriber-specific basis with aPN sequence in such a way that the transmission data in the base stationcan be unambiguously allocated to the subscriber unit. The presentinvention is to utilize the transmission of PN sequences within thescope of a TDMA system for identifying subscriber units during areservation request. In this way, transmission resources of the TDMAsystem are utilized efficiently and transmission delays are reduced.

[0010] Preferably, the PN sequence is an M-sequence. M-sequences areconveniently generated from fed-back shift registers with XOR logicoperation in the feedback branch. They can be used in differentreservation methods.

[0011] Advantageously, the PN sequence is a “preferred gold” sequence.This is conveniently obtained by performing an XOR logic operation ontwo M-sequences which vary in phase with respect to one another.

[0012] In a further embodiment, the PN sequence is a “Katsami” sequence.Katsami sequences of different lengths and with different properties areknown such that subscriber units can unambiguously be distinguished fromone another.

[0013] In a further particularly advantageous embodiment, the PNsequence is an orthogonal gold sequence. Such sequences are “preferredgold” sequences which have been lengthened by one element. It is alsopossible to carry out a selection in respect of the orthogonal goldsequences.

[0014] Preferably, the subscriber units transmit at least partiallydifferent PN sequences at at least partially different times, andsubscriber units are identified using the transmitted PN sequence andthe reception time of the PN sequence. In this way, it is possible, fora plurality of subscriber units to use the same PN sequence since thereception time of the PN sequence can be used to unambiguously identifythe subscriber unit. Alternatively each subscriber unit can have aunique PN sequence and subscriber units can be identified at the basestation from the PN sequence alone.

[0015] In a further embodiment, the subscriber units transmit at leastpartially identical PN sequences at at least partially different times,and subscriber units are identified using the reception time of the PNsequence. Within the scope of this sequence-timing method it is, forexample, possible for all the subscriber units to use the sameM-sequence, but to broadcast them at staggered times. The identificationof subscriber units can then be carried out solely by means of thereception time of the PN sequencers.

[0016] In yet a further embodiment, the subscriber units transmit PNsequences at different times and/or with different phases, andsubscriber units are identified by using the reception time of the PNsequence and/or the phase. Here too, it is possible for all thesubscriber units to use the same PN sequence. Since different subscriberunits use partially different times for transmitting the PN sequences itis possible to provide the same phase for a plurality of subscriberunits.

[0017] In a further advantageous embodiment, the subscriber unitstransmit PN sequences during the normal transmission operation, whereinthe PN sequences lie below the noise level of the normal transmissionoperation, and the subscriber units are identified using the receptiontime. The signalling takes place, for example, using a long M-sequencebelow the noise level in parallel with the normal data transmission. Asa result of this the signal-to-noise ratio (SNR) of the normaltransmission is degraded, but, by means of suitable assumptions withrespect to the accessing subscriber units, it is possible to calculatethe level at which the noise power caused by the signalling can lie.

[0018] In a particularly preferred embodiment and in the case of thesequence-time method and in the case of the sequence-time-phase method,the transmission times lie within one timeslot. A timeslot is thereforedivided into a plurality of sub-timeslots with the result that the timeinformation used by the method to identify subscriber units can beacquired from the identification of the sub-timeslot.

[0019] Alternatively, the transmission times can lie within a pluralityof timeslots. In such a method the transmitted information can then beencoded so that even small SNR values permit a low error rate to be madeavailable by means of the coding gain.

[0020] Furthermore, in the case of the sequence-timing method and in thecase of the sequence-level method, a plurality of modulated sequencesare transmitted in succession. Coding is thus also possible within thescope of these methods.

[0021] According to a second aspect of the invention there is provided asystem for reserving timeslots in a TDMA system comprising: a pluralityof subscriber units (CPE) that use the same radio channel fortransmission during different timeslots to a base station, and in whichthe base station administers the radio channel centrally, which ischaracterized by, if appropriate, the subscriber units transferring atransmission request to the base station using a PN (pseudo-randomnoise) sequence.

[0022] In this way, the aforementioned advantages of the methodaccording to the invention are implemented in a system for reservingtimeslots.

[0023] Advantageously, the PN sequence is an M-sequence.

[0024] Preferably, the PN sequence is a “preferred gold” sequence.

[0025] In a further embodiment, the PN sequence is a “Katsami” sequenceor an orthogonal gold sequence. The various PN sequences mentioned abovecan provide different advantages depending on the method applied.

[0026] Furthermore, the system further comprises the subscriber unitstransmitting at least partially different PN sequences at at leastpartially different times, and identifying subscriber units using thetransmitted PN sequence and the reception time of the PN sequence.

[0027] Alternatively, the subscriber units transmit at least partiallyidentical PN sequences at at least partially different times, andsubscriber units are identified using the reception time of the PNsequence.

[0028] It is also envisaged that the subscriber units transmit PNsequences at different times and/or with different phases, and forsubscriber units to be identified using the reception time of the PNsequence and/or the phase.

[0029] In one further advantageous embodiment of the system, thesubscriber units transmit PN sequences during the normal transmissionoperation, wherein the PN sequences lie below the noise level of thenormal transmission operation, and subscriber units are identified usingthe reception time. To optimise the signal-to-noise ratio the PNsequence are advantageously long sequences.

[0030] In the case of the sequence-time system and in the case of thesequence-time-phase system, the transmission times advantageously liewithin one timeslot. Such a timeslot is divided into a plurality ofsub-timeslots so that the time information used to identify subscriberunits can be acquired from the identification of the sub-timeslot.

[0031] Alternatively, in the case of the sequence-time system and in thecase of the sequence-time-phase system, the transmission timesadvantageously lie within a plurality of timeslots. In this way codingof the information is possible.

[0032] In the case of the sequence-timing system and in the case of thesequence-level system, a plurality of modulated sequences are preferablytransmitted in succession. In these methods it is thus also possible toencode the information.

[0033] The present invention is based on the recognition that it ispossible to signal a transmission request to the base station in theform of a pseudo-random noise (PN) sequence, and the base station beingcapable of detecting and unambiguously identifying the transmittingsubscriber unit that has transmitted the PN sequence. A method and asystem are thus made available which only require a small amount oftransmission resource for making reservation requests, whilst ensuring ashort transmission delay. The more efficient use of the channel enablesmore data to be transmitted, which reduces the transmission costs perbit. The short transmission delay improves the quality of thetransmission, for example in the case of voice transmission.

[0034] Embodiments of the present invention will now be explained by wayof example only with reference to the accompanying drawings, in which:

[0035]FIG. 1 shows a schematic view of an ISO OSI model for classifyingthe present invention within an entire communications system;

[0036]FIG. 2 shows an MAC frame with 64-QS(OG-3) sequences in order toclarify a sequence-time method or a sequence-time system according tothe present invention;

[0037]FIG. 3 shows an MAC frame with a 127-M-sequence in order toclarify a sequence-timing method or a sequence-timing system accordingto the present invention;

[0038]FIG. 4 shows an MAC frame with a 255-M-sequence in order toclarify a sequence-time-phase method or a sequence-time-phase systemaccording to the present invention;

[0039]FIG. 5 shows an MAC frame with M-sequences in order to clarify asequence-level method or a sequence-level system according to thepresent invention;

[0040]FIG. 6 shows level diagrams in order to clarify a signal-levelmethod or a signal-level system according to the present invention, and

[0041]FIG. 7 shows a diagram representing error probabilities as afunction of the signal-to-noise ratio for binary signals.

[0042] In order that the invention can be better understood, the basicproperties of various PN (pseudo-random noise) sequences will firstly begiven in the following table. Here, in each case the sequence length,the number of available sequences, the autocorrelation and thepeak-cross-correlation are given for the individual sequence types.Quasi-synchronous timing of the reception sequences in the receiver is aprecondition for the indication of autocorrelation and ofcross-correlation for the QS(OG-3) sequences, that is to say an offsetwithin ±1.5 symbol for QS(OG-3) and ±0.5 symbol for QS(OG-1). TheM-sequences are obtained from fed-back shift registers with XOR logicoperation in the feedback branch. The “preferred gold” sequences areobtained by means of an XOR logic operation performed on two M-sequenceswhich vary in phase with respect to one another. QS(OG-1) sequences areorthogonal gold sequences (“preferred gold” sequences which have beenlengthened by one element). By means of selection, QS(OG-3) sequencesare obtained from the QS(OG-1) sequences. Peak-cross- Sequence LengthNumber Autocorrelation correlation M-sequence 31 6   31/−1  11M-sequence 63 6   63/−1  23 M-sequence 127 18   127/−1  41 M-sequence255 16   255/−1  95 M-sequence 511 48   511/−1  113 Preferred gold 31 3331/9  9 (29%) Preferred gold 63 65 63/17 17 (27%) Preferred gold 127 129127/17  17 (13%) Preferred gold 255 257 255/31  31 (12%) Preferred gold511 513 511/33  33 (6%) Preferred gold 1023 1023 1023/65  65 (6%)Katsami sequence 63 8 63/9  9 (14%) Katsami sequence 255 16 255/17  17(7%) Katsami sequence 1023 32 1023/33  33 (3%) 4-QS(OG-1) 4 4 4 08-QS(OG-1) 8 8 8 0 16-QS(OG-1) 16 16 16 0 32-QS(OG-1) 32 32 32 032-QS(OG-3) 32 8 32 0 64-QS(OG-3) 64 16 64 0 128-QS(OG-3) 128 32 128 0256-QS(OG-3) 256 64 256 0 512-QS(OG-3) 512 128 512 0 1024-QS(OG-3) 1024256 1024 0

[0043] The different methods that will be described below are subject toperipheral conditions which have been assumed by way of example to be asfollows: active CPEs in one sector: maximum of 256 symbol rate (uplink):12.6 M-symbols/s MAC frame length: 1 ms (= 12600 symbols) SNR in thereceiver: approx. 5 dB

[0044] In addition, it has been assumed that for the exemplarydescription of the various methods eight simultaneous reservationrequests have been made.

[0045]FIG. 1 shows an overview of an ISO OSI model in order to be ableto classify the present invention within the entire system. The ISO OSImodel comprises a data link layer 10 and a physical layer 12. Thephysical layer 12 is divided into two layers, specifically a physicallayer convergence protocol (PLCP) 14 and into a physical mediumdependent (PND) layer 16. The data link layer 10 is also divided intotwo layers, specifically into a logic link control 18 (LLC) and into alayer for medium access control 20 (medium access control layer; MAClayer). The methods and systems described within the scope of thepresent invention are to be assigned to the MAC layer 20 within theentire system illustrated in FIG. 1.

[0046] The sequence of a plurality of MAC frames is represented intabular form below: 12600 symbols 12600 symbols 12600 symbols 12600symbols MAC frame N MAC frame N + MAC frame N + MAC frame N + 1 2 3 1 ms1 ms 1 ms 1 ms

[0047]FIG. 2 shows a MAC frame with 64-QS(OG-3) sequences in order toillustrate a sequence-time method or a sequence-time system according tothe present invention. Examples of sequences which can be used withinthe scope of the sequence-time method and their properties and theireffect on the transmission are represented in the following table.Timeslot Computational Sequence (overall work per MAC Sequence numberlength) SNR frame 1024-QS(OG-3) 256 1030 35 dB 256 * 1024 * 5 512-QS(OG-3) 128 1033 32 dB  2 * 128 * 512 * 5  256-QS(OG-3) 64 1039 29dB  4 * 64 * 256 * 5  128-QS(OG-3) 32 1051 26 dB  8 * 32 * 128 * 5 64-QS(OG-3) 16 1075 23 dB  16 * 16 * 64 * 5  32-QS(OG-3) 8 1123 20 dB 32 * 8 * 32 * 5  16-QS(OG-3) 4 1219 17 dB  64 * 4 * 16 * 5   8-QS(OG-3)2 1411 14 dB 128 * 2 * 8 * 5

[0048]FIG. 2 illustrates a MAC frame 210 which is divided into aplurality of timeslots 212, 214, 216, . . . , 218. The timeslots 214,216, . . . , 218, of which three are illustrated in FIG. 2, are used for“normal” data transmission. The timeslot 212 makes available 256subscriber units CPE 0, CPE 1, . . . , CPE 255 for dispatchingreservation requests. For this purpose, the timeslot 212 is divided into16 sub-timeslots 220, 222, 224, . . . , 226, of which four arerepresented in FIG. 2. Each of the sub-timeslots 220, 222, 224, . . .226 has a length 228 of 16 symbols. Within an individual sub-timeslot,the subscriber units 16 are assigned different codes C 0, C 1, C 2, . .. C 15. For example, the subscriber units CPE 0, CPE 16, CPE 32, . . . ,CPE 240 are assigned the code C 0. The subscriber units are, however,distinguishable from one another when the code C 0 is received by thebase station because each of the subscriber units CPE 0, CPE 16, CPE 32,. . . , CPE 240 is assigned to a different sub-timeslot 220, 222, 224, .. . , 226. Another code, for example the code C 11 is assigned to theother subscriber units, for example the subscriber units CPE 11, CPE 27,CPE 43, . . . , CPE 251.

[0049] The high SNR values given in the table above result from thefavourable cross-correlation function of the respective sequences.

[0050] In contrast to the illustration in FIG. 2, it would, for example,also be possible to operate with a 256-QS(OG-1) code with in each caseone sequence for the 256 subscriber units. The timeslot length wouldthen be a total of only 256 symbols. However, with a receptioninaccuracy of ±1 symbol severe SNR degradation is then possible underunfavourable conditions. To remedy this, the reception inaccuracy couldthen be reduced to ±0.5 symbol.

[0051]FIG. 3 shows an MAC frame with a 127-M-sequence in order toillustrate a sequence-timing method or a sequence-timing systemaccording to the present invention. Here, all the subscriber units usethe same M-sequence. The identification of the respective subscriberunit is carried out in the receiver using the reception time.

[0052] In the following table, possible M-sequences to be used, andtheir properties and effects on the system, are given. In the SNRcalculation it has been assumed that the autocorrelation function of thesequence used is always −1 beyond the maximum. This applies toperiodically propagated M-sequences. However, as the M-sequences are notperiodically propagated here, when there is overlap of a plurality ofsequences a smaller SNR is obtained. In order to counteract this, it ispossible to use more suitable sequences. Timeslot Computational Sequence(overall work per MAC Sequence number length) SNR frame 1023-M-sequence1 1023 + 3 * 255 21.5 dB 256 * 1023 * 5  511-M-sequence 1  511 + 3 * 25518.5 dB 256 * 511 * 5  255-M-sequence 1  255 + 3 * 255 15.5 dB 256 *255 * 5  127-M-sequence 1  127 + 3 * 255 12.5 dB 256 * 127 * 5 63-M-sequence 1  63 + 3 * 255  9.5 dB 256 * 63 * 5

[0053] The MAC frame 310 is divided into a plurality of timeslots 312,314, 316, . . . , 318. Here, the timeslots 314, 316, . . . , 318, ofwhich three are illustrated, are provided for the normal datatransmission. The timeslot 312 is used to transfer reservation requestsfrom 255 subscriber units CPE 0, CPE 1, CPE 2, . . . , CPE 255. Thesequence used is a 127-M-sequence so that the timeslot 312 has a totallength 328 of 892 symbols.

[0054]FIG. 4 shows an MAC frame with a 255-M-sequence in order toexplain a sequence-time-phase method or a sequence-time-phase systemaccording to the present invention. M-sequences of different phases aretransmitted by the various subscriber units. Different times are alsoused for dispatching the sequences so that this also serves as adistinguishing criterion of the subscriber units.

[0055] First, the use of possible sequences will be given by way ofexample in the following table, their properties and their effects onthe entire system also being shown. Timeslot Computational Sequence(overall work per MAC Sequence number length) SNR frame 1023-M-sequence341 1027 21.5 dB 341 * 1023 * 5 (341 CPEs)  511-M-sequence 170 1031 18.5dB 340 * 511 * 5 (340 CPEs)  255-M-sequence  85 785 15.5 dB 255 * 255 *5 (255 CPEs)  127-M-sequence  42 783 12.5 dB 252 * 127 * 5 (252 CPEs) 63-M-sequence  21 795  9.5 dB 252 * 63 * 5 (252 CPEs)

[0056] If the reception time can fluctuate by more than ±1 symbol, thenumber of phases which can be used is reduced so that fewer subscriberunits can transmit within one timeslot. If there is uncertainty of ±4symbols, only 28 subscriber units can then be used in a timeslot oflength 263 symbols.

[0057] The MAC frame 410 illustrated in FIG. 4 is divided into aplurality of timeslots 412, 414, 416, . . . , 418. The timeslots 414,416, . . . , 418, of which three are represented by way of example, areused for the normal data transmission. The timeslot 412 is used todispatch reservation requests. The timeslot 412 is divided into threesub-timeslots 420, 422, 424, 255 subscriber units CPE 0, CPE 1, CPE 2, .. . , CPE 254 are accommodated in the three sub-timeslots 420, 422, 424.The subscriber units which are represented in a same row in FIG. 4, thatis to say for example the subscriber units CPE 2, CPE 87 and CPE 172,use the same phase P6. Subscriber units which are represented indifferent columns in FIG. 4 use different phases, only every third phasebeing used owing to the uncertainty of the reception time of ±1 symbol.

[0058] The method illustrated in FIG. 4 is of interest forcollision-free reservation. Given 85 active subscriber units, only onetimeslot of the length 255 (plus the guard time) is necessary with a255-M-sequence. Given 8 simultaneous access operations, the SNR is thenstill 15.5 dB with a detection error rate which is less than 2˜10⁻⁵.Given four simultaneous access operations, the detection error ratedrops to 10⁻¹⁰.

[0059]FIG. 5 shows an MAC frame with M-sequences in order to explain asequence-level method or a sequence-level system according to theinvention. Signalling is preferably carried out in parallel to thenormal data transmission using a long M-sequence that is below the noiselevel. The identification of the subscriber unit is carried out on thebasis of the time of the access. So that each subscriber unit can accessonce within an MAC frame (12600 symbols), the interval between twosequences must be less than or equal to 49.

[0060] Exemplary sequences, their properties and their effects on theentire system are illustrated in the following table. ComputationalSequence SNR SNR work per Sequence overlap signalling degradation MACframe 4095-M-sequence 84 13 dB 0.5 dB 256 * 4095 * 5 4095-M-sequence 84 7 dB 0.1 dB 256 * 4095 * 5 2047-M-sequence 42 10 dB 0.5 dB 256 * 2047 *5 1023-M-sequence 21  7 dB 0.5 dB 256 * 1023 * 5  511-M-sequence 11  4dB 0.5 dB 256 * 511 * 5

[0061] A MAC frame 510 is shown in FIG. 2 which is divided into aplurality of timeslots 512, 514, 516, 518, . . . , 520, five timeslotsbeing represented by way of example. Normal data transmission takesplace during the timeslots 512, 514, 516, 518, . . . , 520, in whichcase, for example, the timeslot 512 is assigned to the subscriber unitCPE X1 and the timeslot 516 is assigned to the subscriber unit CPE X3.In parallel with the normal data transmission, reservation requests aredispatched in the form of access sequences 522, 524, 526, 528, 530, ofwhich five are represented by way of example in FIG. 5. Each of theaccess sequences has a length of 2047 symbols in the example.

[0062] In FIG. 6, signal levels are shown which occur in conjunctionwith an exemplary method using a 2047-M-sequence, the left-hand side ofthe illustration showing signal levels before despreading and theright-hand side showing signal levels after despreading. The level 610shows an access signal level for a subscriber unit. Level 612corresponds to 8 subscriber units. The level 612 is still 9 dB below thenormal noise level 614, it being assumed for the transmission burst thatit has, with respect to the normal noise level 614, a level 616 with anSNR of 5 dB. After despreading, in which a despreading gain of 33 dB isassumed, an SNR of 10 dB is present with respect to the level 618 of theaccess signalling.

[0063] During the calculation of the SNR degradation, the mostunfavourable combination of subscriber units in the access operationswas assumed, that is to say it was assumed that there were successiveaccess operations. However, as in all the exemplary embodimentsdescribed above, a maximum of 8 access operations is assumed during anMAC frame. For an SNR degradation of 0.5 dB, the noise power caused bysignalling must be 9 dB below the normal noise level.

[0064] It is to be noted that within the scope of the signal-levelmethod and the signal-level system it is possible to achieve a smalldetection error probability, preferably with long sequences.

[0065] The degradation and necessary countermeasures are illustrated ina table below for different paths for multi-path propagation for themethods described above. Strong Strong long-range Method short-rangeechoes echoes Weak echoes Sequence-time High high small methodSequence-timing medium-sized/ medium-sized small method relatively largeintervals Sequence-time- medium-sized medium-sized small phase methodamount/relatively few phases Sequence-level Small small small method

[0066] With the exception of the sequence-level method, all the methodsreact to strong echoes in a sensitive way. With adaptive receivers,partial or complete compensation of multi-path propagation is possiblewith a good SNR.

[0067] Furthermore, it should also be noted that given a known phaseshift or amplitude change between the transmitter and receiver, forexample, as a result of repeated transmission, the transmitter canmodulate the phase or the amplitude with information. It is then alsopossible to use relatively high modulation types (QPSK, N-PSK, N-QAM,etc.). As a result, additional information can be transmitted or thetransmission can be protected by coding.

[0068]FIG. 7 shows a diagram representing error probabilities as afunction of the signal-to-noise ratio for binary signals. Given anidentical probability of occurrence of a reservation request or of theabsence of a reservation request, the probability of error detection isobtained in accordance with FIG. 7. For a high detection probability,large SNR values are necessary, for example 13 dB for 10⁻³ detectionerrors. These values apply to ON/OFF signalling, as is represented bycurve ‘a’. If antipodal levels are used for the transmission of a bit,an error curve which is better by 6 dB is obtained, as is illustrated inFIG. 7 by the curve designated by ‘b’. However, in order to realizethis, reference values (amplitude/phase) must be present.

[0069] The above description of the exemplary embodiments according tothe present invention serves only for illustrative purposes and not forthe purpose of restricting the invention. Various changes andmodifications are possible within the framework of the invention withoutdeparting from the scope of the invention or its equivalents.

1-24. (Canceled)
 25. A method of reserving timeslots in a time divisionmultiple access system having a plurality of subscriber units that use asame radio channel for transmission during different timeslots to a basestation, and in which the base station administers the radio channelcentrally, comprising the steps of: if appropriate, the subscriber unitstransferring a transmission request to the base station in a form of apseudo-random noise (PN) sequence.
 26. The method according to claim 25,in which the PN sequence is an M-sequence.
 27. The method according toclaim 25, in which the PN sequence is a preferred gold sequence.
 28. Themethod according to claim 25, in which the PN sequence is a Katsamisequence.
 29. The method according to claim 25, in which the PN sequenceis an orthogonal gold sequence.
 30. The method according to claim 25,and further comprising the steps of: the subscriber units transmittingat least partially different PN sequences at at least partiallydifferent times, and identifying subscriber units using the transmittedPN sequence and a reception time of the PN sequence.
 31. The methodaccording to claim 25, and further comprising the steps of: thesubscriber units transmitting at least partially identical PN sequencesat at least partially different times, and identifying subscriber unitsusing a reception time of the PN sequence.
 32. The method according toclaim 25, and further comprising the steps of: the subscriber unitstransmitting PN sequences at different times and/or with differentphases, and identifying subscriber units using a reception time and/orphase of the PN sequence.
 33. The method according to claim 25, andfurther comprising the steps of: the subscriber units transmitting PNsequences during normal transmission operation, wherein the PN sequenceslie below a noise level of the normal transmission operation, andidentifying the subscriber units using a reception time of the PNsequence.
 34. The method according to claim 32, in which for thesequence-time steps and for the sequence-time-phase steps, thetransmission times lie within one timeslot.
 35. The method according toclaim 32, in which for the sequence-time steps and for thesequence-time-phase steps, the transmission times lie within a pluralityof timeslots.
 36. The method according to claim 33, in which for thesequence-time steps and for the sequence-level steps, a plurality ofmodulated sequences is transmitted in succession.
 37. A system forreserving timeslots in a time division multiple access system having aplurality of subscriber units that use a same radio channel fortransmission during different timeslots to a base station, and in whichthe base station administers the radio channel centrally, comprising: ifappropriate, the subscriber units transfer a transmission request to thebase station in a form of a pseudo-random noise (PN) sequence.
 38. Thesystem according to claim 37, in which the PN sequence is an M-sequence.39. The system according to claim 37, in which the PN sequence is apreferred gold sequence.
 40. The system according to claim 37, in whichthe PN sequence is a Katsami sequence.
 41. The system according to claim37, in which the PN sequence is an orthogonal gold sequence.
 42. Thesystem according to claim 37, wherein the subscriber units transmit atleast partially different PN sequences at at least partially differenttimes, and identify subscriber units using the transmitted PN sequenceand a reception time of the PN sequence.
 43. The system according toclaim 37, wherein the subscriber units transmit at least partiallyidentical PN sequences at at least partially different times, andidentify subscriber units using a reception time of the PN sequence. 44.The system according to claim 37, wherein the subscriber units transmitPN sequences at different times and/or with different phases, andidentify subscriber units using a reception time and/or phase of the PNsequence.
 45. The system according to claim 37, wherein the subscriberunits transmit PN sequences during normal transmission operation, andwherein the PN sequences lie below a noise level of the normaltransmission operation, and identify the subscriber units using areception time.
 46. The system according to claim 44, in which for thesequence-time systems and for the sequence-time-phase system, thetransmission times lie within one timeslot.
 47. The system according toclaim 44, in which for the sequence-time system and for thesequence-time-phase system, the transmission times lie within aplurality of timeslots.
 48. The system according to claim 45, in whichfor the sequence-time system and for the sequence-level system, aplurality of modulated sequences is transmitted in succession.